The present invention relates in general to a modulation and demodulation technique for use in, as examples, broadcasting and communication-related applications. More particularly, the invention is directed to an improved manner of synchronizing a combined wave signal in an orthogonal frequency division multiplexing (OFDM) method. The present invention also relates to an OFDM modulator and demodulator and a quadrature amplitude modulation (QAM) modulator and demodulator for implementing the improved synchronization method.
In a transmitting station of a conventional OFDM system, group of a digital signals within a certain time period are amplitude-modulated (hereinafter, called AM-modulated) with a plurality of carriers. The carriers are then combined, and modulated onto a main carrier, for example, a channel-2 carrier. The modulated result is finally transmitted to a receiving station. In the receiving station, the main carrier is detected and the OFDM combined wave is fast-Fourier-transformed, to thereby demodulate a desired digital signal. The transfer of data between the transmission and receiving stations requires synchronization for proper demodulation of the combined wave. Therefore, it is necessary to use a synchronization signal to properly synchronize clocks at the transmission station with the clocks at the receiving station.
In conventional OFDM synchronization methods, the transmission station and receiving station are not synchronized sufficiently to enable stable operations when noise is contained in the carrier. That is, the transmission station does not modulate and transmit a radio frequency (RF) carrier at the first symbol period in a frame, while the receiving station phase locks to the carrier during the non-modulation interval of time, to synchronize with the transmission end. Thus, according to the above conventional method, a reference signal locked to the carrier within the non-modulation time interval, becomes unstable due to generation phase errors in the carrier when noise is contained in the carrier. This is a problem and disadvantage of the conventional OFDM system.